1. Field of the Invention
The present invention relates to the processing of tea leaves to produce black tea.
2. Background Information
The present invention relates to oxidation of green tea leaves to black tea. As is accepted in the art, green tea is tea which has been freshly picked and which generally has undergone treatment, such as a heat treatment, to inactivate enzymes contained in the tea which oxidize chemical substances contained in the tea.
Black tea is prepared conventionally by subjecting freshly picked tea leaves to various processing steps which include a fermentation step which employs enzymes naturally present in the tea to effect enzymatic oxidation of chemical substances contained in the tea which results in providing the organoleptic and aesthetic characteristics, i.e., aroma, flavor and color, associated with aqueous beverage extracts obtained from black tea. Extracts of black tea may be consumed as a hot beverage or may be chilled to provide a cold beverage, or the extracts may be processed further to provide an instant water-soluble product for preparation of hot and cold beverages.
Although beverages prepared from green and Oolong teas are appreciated by consumers in various parts of the world, in some localities, particularly in the United States, consumer preferences dictate that tea beverages have the organoleptic and aesthetic characteristics of beverage extracts obtained from black tea. In contrast to the distinctive reddish coloration of extracts obtained from black tea, aqueous extracts obtained from green tea, in particular, have a yellow-greenish coloration which tends to reinforce a perception in consumers that the extracts have a xe2x80x9cgrassyxe2x80x9d flavor and aroma and a xe2x80x9cbitterxe2x80x9d taste. Oolong teas have organoleptic and aesthetic characteristics which fall in between those of green and black teas.
Seltzer discloses a process said to enable obtaining fermented black tea and partially fermented tea of more uniform quality from green tea. To obtain this objective, the process is carried out by extracting green tea leaves with water and then combining the aqueous extract with what is described as a xe2x80x9crelatively small amountxe2x80x9d of xe2x80x98bruisedxe2x80x99 fresh tea leaves and then heating the mixture of the extract and bruised leaves in the presence of oxygen at a temperature not to exceed 43xc2x0 C. for a period of time after which the reaction mixture is heated to inactivate the enzymes. The extract obtained is said to have characteristics of black tea.
Gurkin discloses treating an aqueous extract of green tea in the presence of oxygen or treating green tea leaf in the presence of water and oxygen at a temperature above 50xc2x0 C., and preferably, at a temperature of from 75xc2x0 C. to 125xc2x0 C., under a pressure of at least 100 psig (7.03 kg/cm.sup.2), and preferably at a pressure of from 14.06 kg/cm.sup.2 to 56.24 kg/cm.sup.2. Times of treatment may range from 2 mins to 30 mins. It is taught that, preferably, the pH of the reaction media be above a pH of 7 prior to the treatment. In addition to demonstrating the effects of variables of pressure, time, pH and concentration of tea solids when treating aqueous extracts, Gurkin discloses treating macerated leaves in water in a ratio of water to leaf of 9:1. Gurkin also posits that the treatment may be carried out by adding a xe2x80x9csmall amountxe2x80x9d of water to the green leaf and converting it to black tea leaf under the disclosed conditions.
Moore, which was assigned commonly with Seltzer and Gurkin, also discloses a process for treating water-soluble constituents of green tea leaves, particularly aqueous extracts thereof, in the manner of Gurkin. Moore, however, differs from Gurkin in that the process requires that the reaction media have a pH of at least 7.5. It is disclosed that such a pH was found to be a xe2x80x9cmajorxe2x80x9d factor affecting the color of the final product and that such a pH is necessary to produce a xe2x80x9cpractical degree of conversion within a commercially feasible timexe2x80x9d. When leaves are to be treated, Moore teaches that they are to be treated in an alkaline solution in which the majority of the solution is absorbed by the leaf.
It is an object of the present invention to provide an improved black tea manufacturing process through the use of the product of which produces a beverage of enhanced therapeutic or health enhancement qualities.
It is an object of the present invention to provide an improved black tea manufacturing process through the use of the product of which produces a beverage which elevated, beneficial theaflavins, when compared with beverages made from conventionally prepared black tea.
Mushroom tyrosinase rapidly reacts with dioxygen and then release oxygen and become deoxytyrosinase by oxidation of phenols or polyphenols. In fact tyrosinase is a catalyst in oxidation of phenols.
The present invention is characterized in that polyphenolic substances contained in moist fresh tea leaves having a moisture content of at least 20% by weight based upon the dry weight of tea solids contained in the moist leaves (xe2x80x9cby weight dry tea solidsxe2x80x9d) are oxidized at about 20-50xc2x0 C. by spraying tyrosinase solution on them. For dry green tea, tyrosinase solution is used to soak the leaves. The oxidation duration is 10 to 360 minutes at a good air flow environment or supplying oxygen.
The reaction is characterized further in that the moist leaves are contacted with tyrosinase and molecular oxygen sufficient to the moist leaves to oxidize polyphenolic substances contained in the moist leaves. The treated leaves may be processed immediately for drying to obtain black tea, or preparing water-soluble instant tea products, or they may be dried for subsequent extraction for preparation of a beverage.
It has been discovered that the amount of moisture employed in the oxidizing treatment of the present invention is a very critical variable. By reason of the amount of moisture employed in the process of the present invention, the leaves to be treated are only moist which is believed to facilitate the permeation of the leaves by the oxidizing agent. Thus, in the process of the present invention, by reason of the leaves being only moist and thereby having almost no free water present, the oxidation reaction takes place in localized sites on and within the tea leaf structure. Therefore, the pH changes which occur due to the oxidation reaction occur substantially only at the localized sites and do not substantially affect neighboring oxidation reaction sites, which would occur if free water were present.
In carrying out the process of the present invention, the moist tea leaves, and hence the various oxidizable substances of the tea leaves, are contacted with molecular oxygen. Although various oxidizing agents can be employed to provide the molecular oxygen for the reaction, various chemical agents such as hydrogen peroxide or permanganate, for example, may be deemed undesirable because residues of the same in the treated leaves may be considered to be food additives, and thus, the final product therefore would not be considered to be 100% tea. Thus, gaseous oxidizing agents are most preferred.
Suitable gaseous oxidizing agents include ozone or ozone-containing gases, but more advantageously, an oxygen-containing gas, including air and oxygen enriched air may be employed. For most efficient results, however, oxygen gas is employed as the oxidizing agent.
When the tea leaves to be treated in accordance with the present invention are in a dry state, having a stable moisture content which conventionally is in a range of from about 5% to about 7% by weight dry tea solids, the leaves first are moistened with water solution of tyrosinase, conveniently in the vessel in which the oxidation step is to be performed. The moistening step should produce moisture contents no greater than approximately 40% by weight of dry tea solids. Higher levels of moisture should be avoided since such levels generally will result in saturation of the leaves and the presence of free water, the latter adversely affecting the quality of the oxidation reaction as discussed elsewhere herein.
The object of the moistening step is to moisten the leaves uniformly, and to that end, preferably, the leaves are moistened by spraying them with water by means such as with a series of nozzles contained in the vessel. Preferably, the leaves being moistened are agitated, such as with a stirrer, by tumbling, or by a fluidized bed, or other such agitating means. It would be preferred that the moist leaves and molecular oxygen be contacted in a countercurrent flow. For best results, after adding water solution of enzymes to the leaves, the moisture is allowed to equilibrate throughout the leaves, preferably while agitating the leaves, so that the moisture is substantially uniformly imbibed by and distributed in the leaves and so that there is no free water between and amongst the moist leaves to be treated in the oxidizing step.